In recent years, optical disks have attracted attention as an information recording medium having a large storage capacity and permitting random access, and have come remarkably into wide use. Such optical disks are grouped into a read only type, a write-once type and a rewritable type. Read only type optical disks are applied, e.g. to video disks, compact disks, etc., and have formed a large-scale market. On the other hand, write-once type and rewritable type disks are applied, e.g. to document, computer and video files. And, development thereof is still under way, although some of them are already on the market. Substrates for such optical disks are required to have excellent properties such as low birefringence, high transparency, high heat resistance and high resistance to moisture absorption. In particular, write-once type and rewritable type optical disks are required to be excellent in the above properties as compared with read only type disks.